Timing of Sample Collection

Samples collected after fasting or in the early morning may show highest concentrating ability and highest yield of sediment. The exception to this is the cat with access at all times to a litter box. This sample is also least likely to show glucosuria. Cytologic quality of the cells will be altered by prolonged exposure to waste products, osmolality, and pH variations.

Recently formed urine provides better cytologic detail, and bacteria are more easily identified. If the sample is dilute, tubular concentrating ability cannot be evaluated. Dilute samples also may distort cells.

Collection Technique

The most reliable method for collecting urine from cats is by cystocentesis. Cystocentesis samples reflect prerenal, renal, ureteral, and bladder health. Voided samples reflect the aforementioned, as well as the urethra, prostate, vagina, and perineal fur. Further, voided samples reflect where the cat has urinated (e.g., the litter box, consultation table, or floor). The yield of the sediment must also be interpreted in light of collection technique. The bladder contracts circumferentially; however, sediment depends on gravity. Thus, for a cystocentesis-collected sample, the sediment yield may be improved by gently shaking the bladder just before inserting the needle.¹⁷⁸ Voided samples also do not reflect sediment proportionately because of sediment remaining in the bladder as it contracts; thus samples collected in this manner may underestimate the degree of inflammation, crystalluria, and so on.

Cystocentesis: The bladder must contain a sufficiently large volume of urine for the veterinarian to be able to identify it by palpation and immobilize it manually. The two approaches used are either through the lateral abdominal wall with the cat on its side or ventrally with the cat in dorsal recumbency. Ideally, the hair is shaved and the skin disinfected; however, as this adds to stress for the patient, it is often not performed. After the bladder is agitated, the needle should be inserted in the caudoventral direction on an angle so that the layers of the bladder wall seal the puncture better. By using a smaller-gauge needle (e.g., 23 to 25 G) and not applying pressure to the bladder with the mobilizing hand, the veterinarian can reduce the risk of urine leakage.

If a swirl of blood is seen to enter the hub of the syringe, collection should be discontinued and the blood (an iatrogenic cause for hematuria) should be noted in the medical record. This bleeding is extremely unlikely to result in postcollection complications, however. Iatrogenic hematuria is commonly seen in cystocentesis samples and may be differentiated from true hematuria by comparison with a free catch–voided sample collected by the client at home 24 to 48 hours later. Clients can use a long-handled spoon such as a soup ladle or put clean aquarium gravel or Nosorb in a clean, empty litter box to collect the sample. Penetration of a bowel loop during cystocentesis is unlikely to cause problems other than in the interpretation of bacteriuria (discussed later). The most disconcerting postprocedural complication is the rare occurrence of vomiting and hypotensive collapse. Although the mechanism is unclear, it is believed to be a vasovagal response. With standard fluid therapy (to support volume and systemic blood pressure [BP]) and quiet, patients recover within 30 minutes to 1 hour.

Voided sample: Midstream collection reduces the chance of collecting debris (particulate material including feces and bacteria) from the perineal region; however, a voided sample is never completely free from risk of contamination. Veterinarians often make do with samples collected from the examination table, examination room floor, cage, litter box, or carrier base. These may also be used as long as the veterinarian is aware that the sample is likely to include artifacts (and know which types are most likely).

Catheter collection is another possibility; however, it requires sedation in both sexes to ensure humane treatment and minimize potential trauma to the urethra. In the very young kitten (younger than 3 or 4 weeks of age), a urine sample may be obtained by stimulating the anogenital region with a warm moist cotton ball.

Handling the Urine Specimen

In an ideal world urine should be kept at room temperature and evaluated within 30 minutes of collection. Storage time and temperature alter and affect pH, USG, and crystal formation.⁷ If it cannot be examined in this time frame, the following suggestions will help preserve the integrity of the specimen.

Examination of the Sample

To minimize interassay variation, a standardized protocol should be used for every sample. Refrigerated samples should be rewarmed to room temperature before evaluation. Urine strips and other reagents should be kept cool but not refrigerated. These and the urine should not be exposed to sunlight or other light for significant amounts of time. It is important to read and interpret the test results at the times designated by the test manufacturer. Centrifugation of urine sediment should be at 1000 to 1500 rpm for 3 to 5 minutes. Most important, the timing and method of collection should be taken into consideration when interpreting the significance of the results relative to the patient.

Urinalysis Interpretation^177,190

Volume: The normal 24-hour urine volume production for an adult cat is 20 to 40 mL/kg per day. When the USG is greater than 1.040, polyuria is unlikely. Occasionally, cats with renal insufficiency may paradoxically concentrate their urine above 1.040.

Color: Clarity and color are affected by many things, which, in turn, affect the USG value perceived with an optical refractometer. Conversely, urine color should also be interpreted in light of the USG. The color of the sample may be important insofar as it can affect interpretation of the colorimetric dry chemistries (urine strips). Color comparisons are subjective and are affected by colored urine constituents. Color should be assessed by a trained professional, in a consistently well-lit area and using fresh urine (Figure 32-1). Urine color may provide valuable information, including the following:

FIGURE 32-1 Urine color is best assessed in good lighting against a white background.

Turbidity: Transparency is assessed by holding a clear glass tube against a printed page and assessing the legibility of the print. Concentrated urine is more likely to be turbid than dilute urine. Refrigeration changes clarity, as do substances affecting pH. Most commonly, turbidity is caused by sediment—namely, crystals, cells (red blood cells, white blood cells, epithelial cells), bacteria, yeast, semen, or contaminants from the collection container (as well as litter box, carrier, table top, floor) or feces. If there is lipid (from pericystic fat) in the urine, it will rise to the surface of the sample.

Crystal formation is affected by temperature; these may form as urine cools from body temperature to room or refrigerator temperature. Hematuria results in brownish to reddish (rarely black) turbid urine. Myoglobin and hemoglobin create a similarly colored, but clear, urine.

Odor: Cat urine has a characteristic odor that is stronger when the urine is concentrated. Tomcat urine has an almost pathognomonic smell that helps identify an intact cat or a cat that has been incompletely castrated (e.g., retained testicle) or a cat with a testosterone-secreting tumor. It has been speculated that felinine, the amino acid unique to cats, is responsible for this smell.

Abnormal odors may indicate infection with urease-producing bacteria. Warm temperature facilitates transformation of ammonium [NH₄] to ammonia [NH₃], resulting in odor. The odor of urine ketones may be detected by some humans. A putrid smell suggests bacterial degradation of protein.

USG: USG is a measure of the density of the urine relative to the density of water measured at the same temperature. The density of water is 1.000 under set circumstances of temperature and pressure. Temperature affects USG inversely (i.e., increasing urine temperature causes a decrease in its USG, whereas decreasing the urine temperature increases the USG). Solutes affect the density of urine, and each solute may affect it to a different degree, even when each one is present in equal amounts.

The accepted method for determining USG in cats is by using a refractometer. This tool assesses refractive index (ratio of velocity of light in air to the velocity of light in a solution). The refractive index is affected by the type and quantity of solutes present. Although refractometers are calibrated to a reference temperature, they compensate to a certain degree. They should be stored at room temperature. Veterinary refractometers measure a wider range of specific gravity and are therefore best suited for cat urine that may have USG in excess of 1.080. Human refractometers read only to 1.050. Digital refractometers appear to correlate with optical refractometers and have the advantage of less subjectivity.²² Some reagent strips include a pad for USG. Because these are developed for human urine, the highest value they detect is 1.030, which is inadequate for feline urine. Urinometers, devices that float in urine to measure USG, are imprecise. Osmometers assess osmolality rather than specific gravity. Regardless of the method used, all factors that affect refraction still should be taken into consideration.

The normal USG for a cat depends on hydration status and age. By the time a kitten is 4 weeks of age, USG is 1.020 to 1.038; full concentrating ability (up to 1.080) is reached by 8 weeks of age. In a dehydrated cat normal renal function (specifically, concentrating ability), is suggested by USG of 1.040 or above, depending on the diet fed. In cats fed exclusively canned food, a “normal” USG may be 1.025 or greater, whereas in cats fed exclusively dry food, USG should be 1.035 to 1.040 or higher.⁵² In a well-hydrated cat, it may fall between 1.035 and 1.060. Specific gravity varies within the same individual throughout the day; therefore a single urine sample with a low USG is not reliable evidence of a decline in renal function.

When nephrons are no longer able to modify glomerular filtrate, a fixed USG of 1.008 to 1.012 develops. USG of 1.007 to 1.039 in a dehydrated cat with or without azotemia is highly suggestive of renal insufficiency (or renal failure, depending on the degree of azotemia once the patient is rehydrated).¹⁷⁶ Hypoadrenocorticism and hyperaldosteronemia are less common causes of such a drop in urine concentration. There is a subgroup of cats with impaired renal function that paradoxically remain able to concentrate urine to greater than 1.045, such that renal azotemia precedes a decline in USG.¹⁷⁷ Because these patients are uncommonly identified, veterinarians must rely on finding USG of 1.045 or greater in the face of azotemia as indicating a prerenal cause for the azotemia.

Urine pH: Urine pH can be used as an index of body acid–base balance; however, this parameter changes so rapidly to provide homeostatic balance to the body that it is a rough guide at best. Obligate carnivores create a great deal of acidic metabolic waste. They regulate their acid–base balance by excreting hydrogen (H⁺), ammonium ion (NH₄⁺), and phosphates (PO₄⁺) in urine (metabolic route) and by exhaling CO₂ (respiratory route). pH is one of the factors affecting crystal formation and may be manipulated to encourage dissolution of some crystal types. Acidic urine inhibits bacterial growth.

Stress affects urine pH. In one study it was reported that the urine pH of a cat increased by 1.4 U when the cat was transported from home to a veterinary clinic. The authors concluded that the most likely cause was anxiety-induced hyperventilation (excessive panting).⁴³ Another study suggested the opposite—namely, that increasing activity of the sympathetic nerves and the adrenal glands will most likely lead to increased metabolism, including catabolic conversion of proteins, which in turn increases sulfuric acid production and lowers urinary pH. This effect can also be seen in the fasted, inappetent, or anorectic cat.⁶⁰

Eating affects urine pH. Postprandial alkaline tide (alkaline urine) is believed to be a result of increased hydrochloride acid secretion after a meal. In their feral state cats eat many (8 to 15) small meals per day rather than two, as are fed in many homes, making the effect of this pH swing much smaller. Frequency of feeding along with quality of food ingested and the composition of the meal will affect urine pH. Higher-protein, meat- and fish-based diets create more acidic urine; lowerprotein, grain- and vegetable-based diets create more alkaline urine.

The pH of urine in the healthy “normal” cat generally ranges between 6.0 and 7.5. The pH of urine least likely to result in crystal formation is 6.2 to 6.4. The method used to measure urine pH is critical; pH meters are inexpensive and are most accurate. Hydrogen paper (pH 5.5 to 9.0) is satisfactory. The urine reagent strips most commonly used in clinics are extremely unreliable. pH values measured with reagent strips are accurate only to within 0.5 units, meaning that the color subjectively translated into a pH value may vary by +/− 0.5, resulting in one whole unit variability.

Acidic urine may be a result of an acidifying diet, respiratory or metabolic acidosis, diabetic ketoacidosis, renal failure, starvation or anorexia, pyrexia, protein catabolism, hypoxia, or severe diarrhea. Severe vomiting resulting in chloride depletion may cause paradoxical acidosis.

Alkaline urine is associated with an alkalinizing diet; drug therapy; respiratory or metabolic alkalosis; vomiting; renal tubular acidosis; chronic metabolic acidosis resulting in ammonium ion (NH₄⁺) secretion; and infection with urease-producing bacteria, such as Proteus and Staphylococcus, organisms seen infrequently in the urinary tract of cats.

Drugs may alter urine pH. Acidifiers include DL-methionine, furosemide, ammonium chloride (NH₄Cl), ascorbic acid at supertherapeutic doses, and phosphate salts. Alkalinizing agents include sodium bicarbonate (NaHCO₃), potassium citrate, sodium lactate, and chlorothiazide.

Artifacts affecting urine pH include containers contaminated with detergents or disinfecting agents, CO₂ loss resulting from storing urine at room temperature, and contamination of the sample by urease-producing bacteria from the environment or the distal urethra.

Glucose: The glucose pad on a urine strip is a colorimetric test based on glucose oxidase activity. Although it is easy to use, several points are worth noting. Because the test involves multiple enzymatic steps, it must be performed according to label instructions. The colorimetric indicators can react with substances other than glucose, and some substances may inhibit the test; this means that false-positive and false-negative results are possible. Glucose oxidase is labile, so the expiration date of the strips should be respected. The reaction is also pH dependent. Because the test is temperature dependent, the urine has to be tested at room or body temperature.

Glucose is filtered by the glomerulus and reabsorbed by the proximal tubules. Physiologic or stress glucosuria occurs when serum glucose exceeds the renal threshold for glucose of greater than 260 mg/dL (14 mmol/L). Pharmacologic agents that can result in transient glucosuria include epinephrine, phenothiazines, glucagon, adrenocorticotropic hormone (ACTH) and morphine. Persistent glucosuria may be a result of DM, hyperprogesteronemia, acromegaly, hyperadrenocorticism, and pheochromocytoma. Renal glucosuria may be caused by acute tubular injury.

Urine glucose monitoring for insulin dose titration in the diabetic cat should not be used because the relationship between serum glucose concentration and that in the urine is variable.¹⁷⁷

Ketones: Ketones (ketone bodies) are produced when metabolism shifts to using stored fat as a source of energy, such as in cellular starvation (unregulated DM or lack of eating) or when excessive fat is ingested. In other species it also occurs with insufficient carbohydrate metabolism. The three ketones produced are acetoacetic acid, acetone, and beta-hydroxybutyric acid. The first two are detectable in urine using the reagents in urine strips; beta-hydroxybutyric acid is not. All three types can be measured in blood. Another colorimetric reaction on urine strips, ketone pad color interpretation, is subjective and is affected by colored urine constituents.

Bilirubin: Bilirubin is a by-product of heme (from hemoglobin) catabolism. The portion that is bound to albumin (unconjugated/indirect bilirubin) is removed from circulation by the liver where it is conjugated. Once conjugated, it is water soluble. The majority of the conjugated portion is transported in bile to the intestinal tract, where bacteria convert it into urobilinogen. It is oxidized to urobilin, the pigment that provides the brown color to feces. A small amount of the urobilinogen is reabsorbed into circulation and is excreted into urine. The small quantity of conjugated bilirubin that evades the bile is excreted into glomerular filtrate.

An increase in urinary bilirubin is associated with increased destruction of red blood cells (hemolytic disease), hepatocellular disease preventing normal elimination of this product, or bile duct obstruction (cholestatic disease). Altered selective permeability of glomerular capillaries in glomerulonephropathy can also potentially cause bilirubinuria by changing the renal threshold of affected nephrons. Bilirubinuria may precede clinically recognizable icterus and even bilirubinemia. Unlike in dogs, bilirubinuria is not found in normal cats, even in highly concentrated urine samples, presumably because of a higher renal threshold for bilirubin in this species.¹⁷⁷

Bilirubin is an unstable compound, especially when exposed to room air or light. The degradation products formed under those circumstances (including biliverdin) do not react with the test, causing false-negative test results. To avoid this, urine should be evaluated within 30 minutes of collection or be refrigerated, kept dark, and (for other tests) brought to room temperature just before analysis. This test should also be run before centrifugation (or filtration) because precipitates in the centrifuged (or filtered) sample may absorb bilirubin.

Urobilinogen: The reagent strip test detects normal and increased amounts but not the absence of urobilinogen. Because of this, it cannot be used to detect complete bile duct obstruction. Increased concentration is suggestive of hemolytic disease or decreased hepatic function. For results to be meaningful, a fresh urine specimen is required.

Occult blood, hemoglobin, and myoglobin: Hemoglobinuria (red to brown urine) is suggestive of intravascular hemolysis; a serum sample from the patient collected concurrently should have a reddish discoloration. Myoglobinuria (brownish urine) is suggestive of muscle disease; the serum from the patient may be clear.

Free hemoglobin and myoglobin, but not intact red blood cells, cause a positive reaction. This urine strip chemical reaction augments and complements the microscopic findings of red cells on urine sediment evaluation. This test must be interpreted in concert with the USG as well as the microscopic sediment evaluation. Very dilute or very alkaline urine may lyse red cells. Serum creatinine kinase should be assessed when a positive reaction occurs and hemoglobinemia has been ruled out to differentiate between myoglobinuria and hemoglobinuria.

Lack of red cells in the sediment with a positive test reaction implies hemoglobinuria, myoglobinuria, low urine concentration, low pH causing red cell lysis, or misidentification of red cells in the sediment. When red blood cells are seen on microscopic examination but the urine test pad is negative, it suggests that the strips are outdated, the sample was improperly mixed or centrifuged, there are too few red cells in the sediment to hemolyze, or red cells have been misidentified in the sediment.

Hematuria indicates blood loss into any part of the urinary tract. Identification of the site of bleeding is the next step. Idiopathic renal hematuria has been recognized in cats and dogs. It is not known whether it is due to a vascular bed abnormality or to an abnormality in podocyte attachment, as occurs in humans.²²⁰

Protein: Numerous types of protein (as many as 40 kinds) may be found in the urine of cats. Hemoglobin and myoglobin have already been mentioned. Protein detected in urine may be prerenal, glomerular, or postglomerular in origin. Small amounts of protein are routinely found in the urine of healthy individuals, but under normal circumstances plasma proteins included in urine are restricted by size and are 66,000 daltons or lower. Because protein loss may be transient, it is essential to verify that proteinuria is persistent before considering appropriate diagnostics and therapeutics. Sample-to-sample variation may be considerable. Centrifugation removes cells that may be causing positive reactions; therefore if protein is detected on an uncentrifuged sample, the test should be repeated on the supernatant after centrifugation.

The aspects of the glomeruli that determine whether protein leaves the glomerular capillaries are size, electrical charge, and hemodynamics. In general, proteins at or below 45,000 daltons with a positive charge are most likely to pass through. Albumin is 66,000 daltons and has a negative charge, which is why there are negligible amounts of albumin in the urine of a cat with normally functioning glomeruli despite high plasma concentrations (Table 32-2). Plasma hemoglobin is normally bound to haptoglobin, making it too large to cross the glomeruli. When this binding capacity is exceeded, as may occur in hemolysis, then unbound hemoglobin can enter urine.

TABLE 32-2 Examples of Proteins Found in Urine, Their Relative Size, and Implied Reason for Presence

Because tubules reabsorb filtered protein, a great deal of protein has to be lost through the glomeruli, exceeding the capacity of the functional or impaired tubules to reabsorb it, for it to be present in the ultrafiltrate. Some proteins originate from the urinary tract. The distal tubules and collecting ducts secrete Tamm–Horsfall mucoprotein. The urothelium secretes immunoglobulins as necessary (e.g., to protect against ascending infection).

Interpretation of the significance of protein in urine depends on the USG. For example, mild 1+ proteinuria with USG of 1.010 implies greater protein loss than 1+ protein in a sample with USG of 1.040. Localization of the protein source requires knowledge of collection technique and the urine sediment constituents (Table 32-3).

TABLE 32-3 Localization, Causes, and Findings in Proteinuria

Numerous test methods exist to detect urine protein, each having a different specificity and sensitivity. It should be noted that small amounts of protein normally found in urine are not detected by routine methods. When 4+ (approximately 1000 mg/dL) protein is found in the supernatant of a centrifuged specimen, a urine protein : Cr ratio (UPC) should be performed. The UPC should be repeated two to three times at 2-week intervals to verify the persistence of the problem before pursuing additional diagnostics (e.g., biopsy) or therapeutics.

The reader is referred to the ACVIM Consensus Statement¹⁴³ for a comprehensive discussion of causes, significance, identification, and management of proteinuria in dogs and cats. This topic is discussed in greater detail later in this chapter.

Nitrite: In humans this test is used to screen for certain bacterial pathogens. This test is not valid in cats (or dogs).

Leukocytes: In humans this test is used to detect the presence of leukocyte esterase and is a semiquantitative assessment for leukocytes in urine. This test is not valid in cats (or dogs).

Urine sediment: Microscopic examination of urine sediment is akin to exfoliative cytology of the urinary tract. It is critically important in the interpretation of color, specific gravity, turbidity, protein, pH, occult blood, and so forth. Without this procedure it is not possible to differentiate, for example, proteinuria caused by glomerular disease from that of inflammatory response to bacterial insult at any level of the urinary tract or the genital tract. Conversely, the sediment cannot be interpreted without knowledge of the physical and biochemical characteristics of the sample.

To minimize interassay variation, a standardized procedure should be followed. Centrifugation speed for urine sediment is slow: 1000 to 1,500 rpm for 3 to 5 minutes. Faster or longer centrifugation will lead to artifacts. Normal constituents in urine sediment include a few epithelial cells, red and white blood cells, hyaline casts, some fat, mucus, sperm, and some struvite or oxalate crystals. Yeast bodies are contaminants. Abnormal constituents in urine sediment include more than a few red or white blood cells; hyperplastic or neoplastic epithelial cells; more than a few hyaline or granular, cellular, hemoglobin, fatty, or waxy casts; a large number of crystals; any parasite ova; bacteria in a properly collected, transported, and prepared sample; or many yeast organisms.

Storage of urine can alter crystalluria dramatically and therefore the clinician’s diagnosis and treatment planning. A study performed to look at the effects of storage on the diagnosis of crystalluria and casts in cats with no history of urinary tract disease was performed. Crystalluria was detected in at least one aliquot in 92% of stored samples as opposed to only 24% of samples examined fresh. Regardless of whether the sample was stored or fresh, urine from cats fed an exclusively canned diet did not have crystals.²⁰⁸

Pitfalls of interpretation may be avoided by examining unstained sediment using a reduced microscope light intensity. This can be achieved by either lowering the condenser or closing the iris diaphragm. Stain artifact may include bacteria growing in the stain and foreign material. Stain should be filtered weekly or monthly (depending on the number of samples being examined) and should be kept in the refrigerator. Additionally, staining procedures that require washing and counterstaining may result in loss of sediment in the process. Ideally, some of the sediment should be examined unstained, and if there is enough, some of it should be saved for staining. A lack of bacteria on examination does not mean that bacteria are not present.

Microalbuminuria

In humans microalbuminuria is predictive of future renal health. In cats the predictive significance of microalbuminuria is not understood at present. The recommendation of the International Renal Interest Society (IRIS; http://www.iris-kidney.com/) is to continue to monitor this level of proteinuria.⁹⁶

The reader is referred to the ACVIM Consensus Statement¹⁴³ for a comprehensive discussion of the causes, significance, identification, and management of proteinuria in dogs and cats. (This topic is also discussed later in this chapter.)

Polycystic Kidney Disease

Polycystic kidney disease (PKD) is found in Persian, Himalayan, and Exotic Shorthair cats around the world and is reported extensively in the United States,⁵⁸ United Kingdom,⁴⁶ Australia,^17,20 France,¹⁸ Italy,³¹ and Slovenia.⁶⁹ The prevalence rates in these studies are between 40% and 50%. Many young Persian cats are asymptomatic, and renal function may not begin to decline until the cat is 7 or 8 years of age. Other breeds of cats manifest this condition rarely; a case report describing a Chartreux cat was recently published.²¹⁷ It has been shown to have an autosomal dominant mode of inheritance in all of these breeds.^27,153 All affected individuals are heterozygous for the causative mutation; homozygous individuals die in utero. Concurrent with the renal cysts, unilocular or multilocular cysts may be seen in the liver with or without congenital hepatic fibrosis,³³ as well as other abdominal organs (Figure 32-4).⁶⁹

FIGURE 32-4 This necropsy image shows the typical appearance of a polycystic kidney in a Persian cat.

(Courtesy Dr. Susan Little.)

Cats may exhibit no clinical disease, slowly progressive renal insufficiency as adults, or significant disease as young cats, with marked renomegaly. Because of the variable clinical picture and because of the autosomal dominant inheritance in a pedigreed population, screening to identify affected individuals is essential. The condition may be unilateral or bilateral with irregular, enlarged kidney (or kidneys) on palpation. Radiographically, the affected kidney will be irregularly enlarged; on excretory urogram multiple radiolucent areas will be seen. Ultrasound is readily available and may help identify affected cats long before they show clinical signs. Thus sonographic screening is recommended in kittens older than 13 weeks of age; a skilled ultrasonographer can detect cysts in affected kittens as young as 6 to 8 weeks of age. Absence of cysts at a young age does not predict that the kitten will not develop them at a later age. Conversely, a cat with cysts may never show clinical disease. Cysts are located in the medulla or cortex (or both) and are anechoic to hypoechoic, round to irregularly shaped structures of variable size that may grow over time (Figure 32-5). Affected kidneys have indistinct corticomedullary junctions and may have mineralized foci. Further evaluation using intravenous contrast medium allows more definitive identification of cysts with computed tomography (CT), as well as identification of distortion of the renal pelves by cysts.¹⁸⁹

FIGURE 32-5 A, Ultrasound of the left kidney of a cat with polycystic kidney disease. Note the anechogenicity of the cyst. B, This ultrasound is of the right kidney and shows that cysts are of variable sizes within the same cat.

(Courtesy Dr. Edward Javinsky.)

Genetic testing has been developed to detect a C→A transversion at position 3284 on exon 29 of the PKD1 gene, resulting in a stop mutation. A real-time polymerase chain reaction (PCR) assay using fluorescent hybridization probes and melting curve analysis has recently (2009) been developed that may be as reliable but faster than previous methods.⁶³ It is, however, recommended to use both ultrasound as well as genetic testing to improve sensitivity and specificity³² to decrease the prevalence of the disease in the Persian population.⁵⁸ PKD is the leading cause of renal disease in Persian and Persian-related breeds.

Therapy for hepatic and renal cysts is warranted when there is significant compression of adjacent tissue or pain from capsular stretch. Drainage may be performed using ultrasound guidance. In one study of dogs and cats, the drained cysts were infused with alcohol for two periods of 3 minutes. Short-term discomfort was noted in all the patients, with anorexia, lethargy, or vomiting occurring in some.²²⁹

Interestingly, although dogs with renal cysts and humans with PKD have hypertension, cats with PKD are normotensive. One study that looked at the effects of the angiotensin-converting enzyme inhibitor (ACE-I) enalapril on BP, renal function, and the renin-angiotensin-aldosterone system (RAAS) in affected cats compared with healthy controls found that the ACE-I reduced BP in all and resulted in changes in RAAS enzyme activities and hormone concentrations with minimal effects on renal function.¹⁶³

Perinephric Pseudocysts

Much less common than PKD, perinephric pseudocysts (PNPs) surround one or both kidneys, which may be normal or subnormal in size. Not a true cyst, the fibrous sac lacks an epithelial lining; thus the fluid it contains is extravasated rather than secreted. There is no single etiology, and this condition may follow renal trauma (urine leakage resulting in scarification) or perirenal fat necrosis (resulting in inflammation), may be associated with neoplasia (e.g., transitional cell carcinoma¹⁸⁶), or may be categorized as idiopathic. Regardless of the initiating cause, anechoic fluid accumulates between the renal capsule and the parenchyma. The sac is often at a pole, but the fluid dissects between the kidney and its capsule or is extracapsular. Fluid cytology reveals a transudate with urea nitrogen close to that of serum. Because the structure does not communicate with renal parenchyma, contrast does not fill it, and on ultrasound it is seen to envelop the kidney rather than exist within it. One report describes a case in which the pseudocyst communicated with the pleural space, resulting in hydrothorax. Unilateral nephrectomy of the affected kidney resulted in resolution of the hydrothorax.¹⁹¹

Affected cats are generally older (older than 8 years of age); there is no sex or breed predisposition.^21,175 The lesion is initially detected on palpation; renal insufficiency may be diagnosed on the basis of serum biochemistries and urinalysis, either due to associated interstitial fibrosis or the effect of compression. Imaging reveals the nature of the lesion (Figure 32-6).

FIGURE 32-6 A, This lateral radiograph shows a soft tissue density in the region of the kidney. B, This ultrasound of the same patient reveals that the mass is a classic perinephric pseudocyst.

(Courtesy Dr. Edward Javinsky.)

Surgical resection of cyst walls is recommended, although reduction by drainage may provide temporary relief. There is one report of cyst wall omentalization with good long-term outcome.¹⁰⁸ Another case report describes laparoscopic fenestration of the capsule wall with resulting improvement in GFR of the affected kidney; no relapse was seen.¹⁵⁷ In other cases renal disease progresses, with the outcome being related to the severity of the renal disease.